1. Field of the Invention
The present invention pertains to photolithography in a semiconductor fabrication process and, more particularly, to the use of an interdependent binary photomask including multiple, interdependent binary masks during exposure in a photolithography operation.
2. Description of the Related Art
Semiconductor devices, or microchips, are manufactured from wafers of a substrate material. Layers of materials are added, removed, and/or treated during fabrication to create the integrated, electrical circuits that make up the device. The fabrication essentially comprises four operations:                layering, or adding thin layers of various materials to a wafer from which a semiconductor is produced;        patterning, or removing selected portions of added layers;        doping, or placing specific amounts of dopants in the wafer surface through openings in the added layers; and        heat treatment, or heating and cooling the materials to produce desired effects in the processed wafer.Although there are only four basic operations, they can be combined in hundreds of different ways, depending upon the particular fabrication process. See, e.g., Peter Van Zant, Microchip Fabrication A Practical Guide to Semiconductor Processing (3d Ed. 1997 McGraw-Hill Companies, Inc.) (ISBN 0-07-067250-4). The fabrication process generally involves processing a number of wafers through a series of fabrication tools. Each fabrication tool performs one or more of the four basic operations. The four basic operations are performed in accordance with an overall process to finally produce wafers from which the semiconductor devices are obtained.        
Of these four operations, many in the art consider patterning to be the most critical. Patterning is known to those in the art by many names. Other names for patterning include photolithography, photomasking, masking, oxide removal, metal removal, and microlithography. The term “photolithography” will hereafter be used to refer to patterning operations. Photolithography typically involves a machine called an “exposure tool,” or sometimes also called a “stepper” or a “scanner”. An exposure tool positions a portion of a wafer being processed under a “photomask.” The photomask is usually a “reticle,” which is a copy of a pattern created in a layer of chrome on a glass plate. Light is then transmitted through the reticle onto a thin layer of material called “photoresist” previously added to the wafer. The chrome blocks the light while the glass allows it to pass.
The light shining through the pattern on the reticle creates an “aerial image” which, when interfacing with the photoresist at the optimum focal plane, changes the material characteristics of the photoresist where it shines. In essence, this allows the pattern on the reticle to be duplicated in, or transferred to, the photoresist. The change in material characteristics makes the photoresist susceptible to removal in the subsequent develop operation prior to the next sequential process step such as etching or ion implantation. The exposure tool then positions another portion of the wafer under the reticle, and the pattern transfer is repeated. The process is repeated until the entire wafer has completed the pattern transfer operation. This process of shining light through a photomask to treat a photoresist is known as “exposure,” or “pattern transfer.”
The reticle in the example above is more precisely known as a “binary mask” because each portion of the reticle either transmits all the light or blocks all the light. However, ever-decreasing feature sizes have created problems for binary masks. The light shining through the chrome pattern scatters at the edges of the chrome traces, with undesirable effects on the pattern transfer process to the photoresist. The smaller the feature sizes, the more acute the problem. Another problem has to do with a technological limitation known as “depth of focus” (“DOF”), which is also related to the wavelength of light utilized.
Technically, depth of focus describes the ability of an optical system to crisply resolve images in two different focal planes simultaneously. This technological limitation is most commonly encountered in photography. A photographer typically focuses a camera on a subject set against a background. The subject is in a focal plane closer to the camera and the background is in a different focal plane further from the camera. If there is a sufficiently small distance between the subject, i.e., the closer focal plane, and the background, i.e., the further focal plane, the background will be in focus in the resulting photograph. This “sufficient distance” is called the “depth of focus.” Thus, if the second plane is within the first plane's depth of focus, both focal planes may be imaged with a minimal loss of resolution. As the distance of the second plane becomes further from the first, resolution will become poorer until it “blurs,” or becomes out of focus in the photograph.
The photolithography operation is subject to this same limitation. In fact, the depth of focus problem more acutely affects photolithography operations than they do photography because the surface of a wafer typically has many focal planes. The topography of a wafer under fabrication is extremely rugged relative to the wavelength of light utilized in the pattern transfer process. At any given point in the operation, there typically are structures built up and trenches dug into the wafer's surface. The depth of focus limitation is a function of the wavelength of the light employed in the optical system. Photolithography processes employ light having very short wavelengths in order to achieve sufficient resolution of the small features being fabricated on semiconductor wafers. Short wavelengths give a small depth of focus, and as wavelengths become increasingly smaller, so does the DOF. Thus, the distance between the top plane and bottom plane, i.e., the top surface and the bottom of the trenches in the wafer, or even the distance between the top and bottom surfaces of the photoresist, can be sufficiently great as to cause depth of focus problems.
The surface topography combined with the resist thickness frequently dictate that the optimal focal plane for a given feature be very different from a focal plane for another. For example, in some circumstances it may be desirable to focus on the topmost plane whereas in some circumstances it may be desirable to focus on the bottommost plane. Since the wafer is given only a single pass exposure, a focal plane between the top and bottom planes is typically chosen.
Historically, the depth of focus for each of the top and bottom planes is typically deep enough so that they provide an overlap wherein such an intermediate plane can be located. This intermediate focal plane, being inside the depth of focus for both the top and bottom planes, can still provide sufficient resolution. However, in today's advanced applications, the amount of overlap is typically so small that it is difficult or even impossible to locate and maintain the intermediate focal plane consistently enough to provide high yields in a commercial production environment.
One approach to the problem performs the entire photolithography operation twice, using two different binary masks for two different focal planes in two different exposure passes. However, the pattern transfer operation previously discussed is part of a much larger photolithography process. The wafers, once exposed, are then developed, baked, and sent to the next stage in the process flow for either etch or ion implantation. In embodiments where the wafers are exposed twice using different binary masks, the wafers have to be developed, baked, and then subsequently processed after each exposure. However, this “repeated photolithography” approach cannot be used for a number of applications where the features of interest of necessity must be processed simultaneously through subsequent operations.
Another approach to this problem employs what is known as “phase shift” photomasks. There are a variety of phase shift photomask types. But all shift the phase of the light waves so that, when the light scatters, it does not interfere with itself to reduce accuracy and resolution of the pattern in the photoresist. An attenuated phase shift photomask, for instance, comprises a reticle that attenuates the light wave so that only a portion of it transmits through the plate to the wafer. Since no portion transmits all of the impinging light, this type of mask is not “binary.” A complementary phase shift photomask actually comprises two reticles, where, at most, only one of which can be binary. The first is used to expose the photoresist and imprint the pattern in a first pass and the second is used to sharpen the pattern in a second pass. Both passes are performed before the wafer is stepped to process another portion of the wafer so that the wafers are not exposed, developed, baked, and etched twice. However, while this approach effectively provides a small increase in DOF, it is not appropriate for many applications in which this incremental increase is insufficient to cause the focal planes of different feature to overlap.
The present invention is directed to resolving one or all of the problems mentioned above.